Typically, as a nuclear medicinal imaging apparatus, a positron emission computed tomography (PET) apparatus is known. During the imaging performed by a PET apparatus, a radiopharmaceutical agent that is labeled with positron-emitting radionuclides is administered to the subject. As a result, the positron-emitting radionuclides that are selectively incorporated into the body tissues of the subject release positrons, which then get coupled with electrons and are annihilated. At that time, the positrons release pairs of annihilation radiations (hereinafter, referred to as “annihilation gamma-rays) in the direction almost opposite. The PET apparatus detects such annihilation gamma-rays with the use of detectors that are arranged in a ring-like manner around the subject. Then, by referring to the detection result, the PET apparatus generates a time series list (also called “coincidence list”) of coincidence counting information. Subsequently, with the use of the time series list of coincidence counting information, the PET apparatus performs reconstruction and generates a PET image.
Meanwhile, the methods of imaging performed by a PET apparatus include imaging in synchronization with the respiration of the subject. During the respiration-synchronized imaging, an external device other than the main body of the PET apparatus is used to obtain body movement information of the subject. The external device is, for example, a respiratory displacement monitor. Herein, for example, during the imaging, the respiratory displacement monitor emits infrared light to an infrared light reflection marker that is placed on the chest region of the subject; takes photographs using a reflection camera; and traces the marker shadow to obtain information on respiratory displacement. However, in the case of using such an external device, it becomes necessary to separately setup and adjust the external device. Moreover, since the infrared light reflection marker may move out of alignment or may fall down during the imaging, using it can be a complicated task.